Undulating beam energy interchange device



July 5, 1966 R. M. PHILLIPS UNDULATING BEAM ENERGY INTERCHANGE DEVICE Filed on. 18, 1965 INVENTOR M. PHILLIPS ATTORNEY July 5, 1966 R. M. PHILLIPS 3,259,736

UNDULATING BEAM ENERGY INTERCHANGE DEVICE Filed Oct. 18, 1965 3 Sheets-Sheet 3 FlG. 5

WA //A INVENTOR ROBERT M. PHILLIPS ATTORNEY United States Patent M 3,259,786 UNDULATING BEAM ENERGY INTERCHANGE DEVICE Robert M. Phillips, Redwood City, Calif., assignor to General Electric Company, a corporation of New York Filed Oct. 18, 1965, Ser. No. 497,154 Claims. (Cl. 3153) This application is a continuation-impart of a copending application of Robert M. Phillips, Serial No. 194,935, filed May 15, 1962, now abandoned.

This invention relates to apparatus for providing interchange of energy between a stream of charged particles and an electromagnetic Wave and more particularly to improvements in apparatus wherein such interchange of energy is effected by forcing the charged particle stream to travel with an undulatory motion in the presence of the electromagnetic wave.

In a traveling wave tube, an electron stream is exposed .to the electric and magnetic fields of .a traveling electromagnetic wave over an extended region along the axis of propagation of the traveling wave. This exposure of the stream is effected by projecting the electron stream along such axis for a distance equal to several operating wavelengths of the wave. Energy is exchanged between the wave and the electron stream by appropriate adjustment of the relative velocities of wave and stream. In a copending application Serial No. 816,540, filed May 28, 1959, now Patent No. 3,129,356, by Robert M. Phillips and assigned to the assignee of the instant invention, a traveling wave tube is disclosed wherein the electron stream is forced to undulate about the axis of wave propagation in the region of its exposure to the traveling wave. The undulating motion is induced on the stream by a static magnetic field oriented perpendicularly to the axis of wave propagation and alternating as a function of distance along the axis. The undulating stream has an alternating component of velocity transverse to the axis, so that the electrons in the stream will be accelerated or retarded by a component of the electric field of the wave parallel to this (transverse direction of velocity. By adjusting the axial velocity of the stream to have a particular synchronous relationship with the axial phase velocity of the electromagnetic wave an energy-exchanging interaction between electron stream and traveling electromagnetic wave occurs. Thus, the wave may be amplified by extraction by energy from the stream, or

the energy of .the stream may be increased by extraction of energy from the wave; a traveling wave tube employing the former type of interaction is known as a traveling wave amplifier and one employing the latter type of interaction is known as an electron accelerator.

Among the advantages of the above-described invention employing :an undulating electron stream is that an energy-exchanging interaction is possible without eployting a slow-wave structure, such as a helix or loaded waveguide, to slow the axial velocity of propagation of the wave. Elimination of the slow-wave structure eliminates the accompanying disadvantages, such disadvantages including costliness of the slow-wave structure and its low-power handling capabilities. Accordingly, the aforementioned patent application discloses non-loaded rectangular, circular or coaxial waveguides for guiding the traveling electromagnetic wave along an axis for interaction with an undulating electron stream. The electromagnetic waves may have phase velocities greater than the velocity of light .in such non-loaded waveguides.

ln the above-identified patent application the axially periodic magnetic field for inducing nndulatory motion in the electron stream is provided by a series of magnets spaced apart along the axis of propagation of the wave. Within the series of magnets the magnetic poles thereof alternate in sense; i.e., the pole of every second magnet is a north pole and the poles of the interjacent magnets are south poles. In the presence of such an arrangement of magnetic poles, the electron stream traveling along the axis of the traveling wave tube encounters a continuously reversing, or spatially periodic, static component of magnetic field that is directed perpendicularly .to the axis. This static, but spatially periodic, transverse magnetic field component forces the electron stream to undulate periodically about the axis as it travels along the length of the tube, the periodicity of the undulatory path corresponding with the periodicity of the magnetic field.

It has been found that energy can be interchanged between wave and electron stream over a broad frequency range if .a synchronous relationship between wave and stream is established wherein the electron stream pro gresses through a distance equal to one period of the axially periodic static magnetic field (distance along the axis between two like pole pieces) during a time substantially equal to that required for the electromagnetic wave to progress through a distance equal to such magnetic period plus one wavelength of the electromagnetic wave in the waveguide employed. This optimum synchronous relationship is the primary factor determining the changes to be made in tube structure and parameter under different conditions of operation of the tube. Where it is desired to substantially increase the frequency of the electromagnetic wave employed in a tube .of this type, theoretically the optimum synchronous relationship can be maintained if either the velocity of the electron stream is substantially increased or the length of the magnetic period is substantially reduced, or if an effective combination of these two changes is made. However, the velocity of the electron stream cannot be increased beyond the velocity of light. Moreover, in the .ab.ove-described traveling wave tube the velocity of .the electron stream is normally a substantial proportion of the velocity of light, so that to increase the stream velocity substantially beyond this value (requires the expenditure of uneconomical amounts of energy and the employment of unduly high electron accelerating voltages. These high voltages are accompanied by difficult electric insulation problems. Therefore, increasing .the electron velocity is an impractical solution to the maintenance of optimum synchronism where .a higher frequency wave is to be employed. Consequently, optimum synchronism at higher wave frequencies must be effected by reducing the length of the magnetic period.

Theoretically, the magnetic period length may be reduced by spacing closer together the magnets of the structure described. However, although a direct decrease in magnet spacing will preserve the synchronous relationship for increasing wave frequencies, the spacing decrease has attendant disadvantages. Among these disadvantages is the increased tendency for magnetic flux to leak or cross the decreased gap between adjacent magnetic poles without penetrating into the path of the electron stream. Accordingly, the transverse magnetic field component required for beam undulation will be reduced for a particular series of magnetic poles as their mutual spacing is decreased. Additionally, if the magnetic poles constitute the core members of electromagnets, wherein a solenoid is wound around each core member, the reduced spacing between poles may be difficult to obtain without reducing the size of the solenoid. Maintaining the same magnetic field in the core member of a solenoid of reduced size requires a correspondingly increased magnitude of current in the Winding of the solenoid. If the current is not increased as the size of the solenoid is decreased the magnetic field strength provided by the poles is correspondingly reduced. Consequently, decreasing the spacing between the magnets of the magnetic Patented July 5, 1966 r 3 structure described above tends to decrease the transverse magnetic field component available to undulate the electron stream unless difficult, costly, and generally undesirable corrective measures are taken.

An additional consideration in reducing the magnetic period is that if the magnetic field strength is maintained substantially constant as the length of the magnetic period is reduced the necessary amplitude of electron stream undulation is not obtained. It has been found that an adequate transverse electron velocity can be acquired by the undulating stream only if the magnetic field intensity is increased proportionately to the increase in frequency. Thus, not only is it necessary to find a solution to overcome the tendency of the field strength to decrease as the magnets of the above-described structure are brought closer together, but for efficient operation at increasing frequencies of the wave, means must be provided to actually increase the magnetic field strength.

Accordingly, it is the principal object of this invention to provide improved apparatus of the type wherein an interchange of energy is effected between an electron stream traveling with undulatory motion and an elcctromagnetic wave.

Another object of this invention is to provide apparatus of the type wherein interchange of energy is effected between an undulating electron stream and an electromagnetic wave traveling in synchronism therewith'for wave frequencies greater than heretofore obtainable.

Another object of this invention is to provide a magnetic structure of small magnetic period for inducing undulation of an electron stream in a traveling wave tube.

Another object of this invention is to provide a magnetic structure of small magnetic period for a device effecting interchange of energy between an undulating electron stream and an electromagnetic wave of very high frequency traveling in synchronism herewith without consequent reduction in the magnetic field component.

Another object of this invention is to provide an improved magnetic structure for a device effecting inter change of energy between an undulating electron stream and an electromagneticwave of very high frequency traveling in synchronism herewith by providing an increased magnetic field component.

The foregoing objects are achieved by providing, in a traveling wave tube of the type above described, a static periodic magnetic field having a high harmonic content of magnetic field distribution. The improved magnetic structure provided comprises a sour-cc of magnetomotive force disposed to provide a magnetic field directed parallel to the axis of wave transmission and a plurality of magnetic elements immersed in the magnetic field and spaced apart along the axis. This improved magnetic structure provides higher spatial harmonic components and, consequently, shorter harmonic periods of the magnetic field in which the magnetic elements are immersed. In one embodiment of the instant invention, the source of magnetomotive force consists of a pair of opposed spaced apertured magnetic end members, the magnetic field directed parallel to the axis being pro vided between the end members. In this embodiment the magnetic elements are also apertured and are disposed in linear spaced array along the axis between the end members. The resultant magnetic field is shaped by the magnetic elements to have a substantial component perpendicular to the axis of the traveling wave tube at points along the length of the array between adjacent magnetic elements.

It is a further feature of the invention to provide a plurality of angularly spaced longitudinal slots in the wall of the circular interaction waveguide formed by the magnetic elements for the purpose of bringing the maximum electric field of the electromagnetic wave close to the waveguide wall which is the region of maximum undulating magnetic field.

The invention will be described with reference to the accompanying drawings, wherein:

FIGURE 1 is an elevational view, partly in cross-section, of one embodiment of the invention;

FIGURE 2 is a cross-sectional view of the embodiment of FIG. 1, illustrating details of the mode transformer employed;

FIGURE 3 is a cross-sectional view of the embodiment of FIG. 1, illustrating the electric field configuration of the wave transmission mode employed;

FIGURE 4 is a schematic cross-sectional view of a portion of the magnetic circuit of FIG. 1, illustrating the mode of operation of the embodiment; and

FIGURE 5 is a schematic cross-sectional view of a portion of the magnetic circuit of another embodiment of this invention.

The traveling wave tube of FIGS. 1 4 provides an energy-exchanging interaction between an electron stream and a traveling electromagnetic wave in an evacuated waveguide 16. Waveguide 10, of circular cross-section, is adapted to propagate an electromagnetic wave along the direction of axis 11 thereof in a manner well known in the art. An electron gun 13 is disposed to project an electron stream 14, shown pictorially, along axis 11 for interaction with the wave traveling therealong. A plurality of rectangular waveguide sections 16 are coupled to waveguide 10 near one end thereof for launching an electromagnetic wave therealong, waveguide sections 16 receiving input electromagnetic energy through respective gas tight dielectric windows 17. A pluraiity of rectangular waveguide sections 19 are coupled to waveguide 10 near the other end thereof for receiving amplified electromagnetic Waves from waveguide 10, waveguide sections 19 transmitting the electromagnetic energy of these waves to utilization apparatus through respective gas tight dielectric windows 20. Windows 17 and 20 are composed of one of several Well-known materials that are transparent to electromagnetic waves, but which do not allow the passage of gaseous molecules, thereby permitting the maintenance of a vacuum in waveguide 10 and waveguide sections 16 and 19. An electron collector 21 receives the electrons leaving waveguide 10 and dissipates the kinetic energy of such electrons. Collector 21 may be provided with passages for a suitable coolant to flow therethrough to prevent excessive temperature of the collector.

One end of each of Waveguide sections 16 curves toward the central portion of waveguide 10 and is affixed to the outer surface of a circular member 23, which is an extension of waveguide 10. Each of Waveguide sections 16 transfers electromagnetic energy to member 23 and, consequently, waveguide 10 through a respective one of rectangular apertures 24. The size of each aperture 24 is equal to the internal cross-sectional area of waveguide sections 16. By so curving waveguide sections 16, a directional transfer of electromagnetic energy to member 23 is effected, whereby most of the energy transferred from waveguide sections 16 travels along the length of Waveguide 10 and very little of such energy travels toward electron gun 13. The other end of each of waveguide sections 16 is coupled to receive electromagnetic energy from a respective one of channels 26 of a wave divider 27 (FIG. 2), to be described hereinafter.

One end of each of Waveguide sections 19 curves to- Ward the central portion of waveguide 10 and is afiixed to the outer surface of a circular member 29, which is an extension of waveguide 10. Each of waveguide sections 19 receives electromagnetic energy from member 29 and, consequently, waveguide 10 through a respective one of rectangular apertures 39. The size of each aperture 30 is equal to the internal cross-sectional area of waveguide sections 19. By so curving waveguide sections 19, a directional transfer of electromagnetic energy from member-'29, whereby most of the energy traveling along waveguide 10 transfers to Waveguide sections 19 and very little of such energy continues along member 29 beyond waveguide sections 19. The other end of each of waveguide sections 19 is coupled to transfer electromagnetic energy to a respective one of channels 26 and a wave divider 27.

Electron gun 13, one type of electron gun suitable for use with'the instant invention, comprises an indirectly heated cathode 35 and a cathode heater, not shown, mounted immediately behind cathode 35. The cathode heater is connected to a suitable energizing source for heating the cathode 35 to a temperature to emit electrons. A centrally apertured focusing electrode 37 and a correspondingly apertured accelerating anode 38 are provided for projecting the electrons emitted by cathode 35 through circular member 23 and along axis 11 of waveguide 10. Electrode 37 and anode 38 also function to focus the electrons into the concentrated stream 14. An electric potential, not shown, is provided between cathode 35 and anode 38 to project the electron stream along the waveguide with the proper velocity for an energyexchanging interaction with the electromagnetic wave therein. A cup-shaped member 39, supported from a base member 40, the latter being affixed to one end of circular member 23, supports the cathode and anode in proper position to form and project the electron stream.

In the operation of the invention, electron stream 14 interchanges energy with an electromagnetic wave traveling to the right in waveguide 10 of FIGS. 1 and 4. As described in the aforementioned patent application, an energy-exchanging interaction between electron stream and wave is effected in waveguide 10 by forcing the electron stream to undulate about the axis of the waveguide. The undulatory motion is forced on the electron stream by a static magnetic field component oriented perpendicularly to the axis of waveguide 10 and alternating as a function of distance along the length of the waveguide. In the instant invention this static magnetic field component is provided by the novel magnetic structure shown in FIG. 1. This novel magnetic structure includes a magnetic unit which comprises a plurality of axially aligned, sandwiched, apertured members. Alternate ones of the apertured members are magnetic and the remaining members are non-magnetic, the aligned apertures of the sandwiched members defining waveguide 10 through the length of the magnetic unit.

Specifically, in FIG. 1, the magnetic unit comprises a plurality of circularly apertured magnetic discs 42 uniformly spaced apart along the length of the tube and a plurality of circularly apertured non-magnetic discs 43 sandwiched alternately between discs 42. Discs 42 and 43 have the adjacent flat surfaces thereof brazed together in vacuum-type relationship, whereby the aligned cylindrical inner surfaces of the array of discs 42 and 43 comprise the boundary of circular waveguide 10.

The apertures of the discs 42 and 43 are formed with angularly spaced radia'l notches, these notches being longitudinally aligned to form a plurality of angularly spaced longitudinal grooves or slot-s 52 and corresponding lands 53 best shown in FIGS. The longitudinal slots increase the circumferential path length of the wall currents in Waveguide 10 and provide a maximum of the electric field of the electromagnetic wave close to the wall of the waveguide. Since this is also the region of maximum undulating magnetic field, the result is a significant increase in interaction efficiency. (For a waveguide 10 of an internal diameter of about one inch, the slots 52 may be about inch in depth and about 10 degrees wide with the lands 53 being about twenty degrees wide.)

Discs 42 are preferably selected from the class of materials catagorized as soft magnetic materials, this class being generally characterized as those magnetic materials having relatively small coercive force. Such a class .Otf magnetic materials is described by A. E. De Barr in Soft Magnetic Materials Used in Industry, The Institute of Physics, London, 1953 and by P. R. Bardell in Magnetic Materials in the Electrical Industry, Philosophical Library, New York, 1955. The soft magnetic material employed in the instant embodiment is known under the trade name Permendur, which is an alloy :of iron cobalt characterized by a relatively low coercive force but an extremely large value of saturation magnetic induction. The non magnetic discs 43 are composed preferably of copper.

The magnetic unit described is immersed in the magnetic field provided by a source of magnetomotive force, such magnetic field being directed proximate to the axis of waveguide 10 and generally parallel thereto. The

source of magnetomotive force comprises an elongated hollow cylindrical solenoid 45 disposed between a pair of pole pieces 46, the latter being preferably of steel. Pole pieces 46 are disc-shaped and are provided with circular apertures therein, the cylindrical inner surfaces of the pole pieces engaging the cylindrical outer surfaces of the respective magnetic discs 42 disposed opposite ends of the magnetic unit. Solenoid 45 is energized by a suitable source of direct electric current, not shown. In the absence of the magnetic unit, the magnetic field provided by solenoid 45 passes through one of pole pieces 46, emerges from the cylindrical inner surfaces of such pole pieces, extends through the hollow central core of solenoid 45 in a direction substantially parallel to the axis of the solenoid and enters the cylindrical inner surface of the other one of pole pieces 46.

The spaced array of soft magnetic discs 42 shapes the magnetic field provided by the magnetomotive force to have strong components perpendicular to the axis of the tube at points along the length of waveguide 10 between adjacent magnetic discs, as shown in FIG. 4. These perpendicular components are greatest at the outer radius of waveguide 10 and progressively decrease in strength as the radial distance from axis 11 decreases. This shaped magnetic field comprises higher order spatial harmonics, these spatial harmonics having relatively short magnetic periods for operation With electromagnetic waves of very high frequencies. The magnetic field lines are shown in the regions between adjacent ones of the upper halfsections of the magnetic discs 42. Magnetic discs 42, having a very high permeability, function, in effect, as magnetic short circuits for the magnetic flux lines. T here- (fore, very litle magnetic flux is found within the aperture of each magnetic disc. However, in the regions between magnetic discs 42 the non magnetic disc 43 are equivalent to air gaps, so that a substantial fringing of the magnetic flux lines occurs.

The vector sets 48 and 49 illustrate the components of the magnetic field lines which are directed parallel and perpendicular to the axis 11 at a radius less than the inner radius of the magnetic discs. The magnetic field components directed parallel to axis 11 are denoted as axial, or z-com-ponents, the z-direction being indicated at the right end of FIG. 4. The magnetic field components directed perpendicularly to axis 11 are denoted as radial, or r-components, the r-direction being also indicated in the figures. Thus, vector sets 48 and 49 illustrate that the magnetic field between magnetic discs 42 has a substantial component of magnetic field directed radially; i.e., perpendicularly to waveguide axis 11. Vector set 48 illustrates that corresponding radial magnetic field component is directed toward axis 11 and vector sret 49 illustrates that the corresponding radial component is directed away from axis 11. Therefore, a spatially alternating radial component of magnetic field is encountered by the electron stream as it travels along the length of waveguide 10. The curve designated B, represents the fundamental variation of the radial component of the magnetic field as a function of position along the waveguide. Thus, an electron stream traveling down the tube encounters a complete sinusoidal variation in the radial component of magnetic field as it progresses from one disc 42 to another.

It is shown in the aforementioned patent application that if the electron stream, in moving along the waveguide, is forced to undulate so as to have a periodic velocity component parallel to the direction of the electric field component of the electromagnetic wave, an energy-exchanging interaction between stream and wave may occur. FIGS. 3 and 4 illustrate the mode of undu-lation of electron stream 14. The curve designated d represents the path of a representative electron of the stream. The electron undulates on the surface of an imaginary cylinder as it travels along the length of the waveguide, periodieally alternating in the H-direction about a straight line path on the surface of the imaginary cylinder and parallel to axis 1].. (The circumferential, or e-direction is illustrated by the arrow at the center of FIG. 3, and represents displacement along a circular path concentric with respect to axis 11.) This periodic alternation of the electron is due to the electron stream crossing the alternating radial components of static magnetic field as it moves along waveguide so that it is subjected alternately to positive and negative circumferential forces. These tforces induce in the stream periodic B-component of velocity that is perpendicular to the velocity in the axial direction, a complete cycle of the B-component of velocity occuring in a distance equal to that between the center of two adjacent discs 42.

An electric field exerts a force on a charged particle such as an electron, this force taking place in the direction of the electric field. If the electron is moving, an electric field parallel to the direction of motion will either increase or decrease the velocity of the electron, according to the relative direction of the electron and the electric field. If, therefore, each time that segments of electron stream 14 have a maximum component of velocity in the fl-direction the segments are immersed in a decelerating electric field of a traveling wave, the stream velocity will progressively decrease and the stream will give up kinetic energy to the electric field. Accordingly, the wave will be amplified as wave and electron stream progress along the length of waveguide To and the device is termed atraveling wave amplifier. If, on the other hand, the electron stream segments are immersed in an accelerating electric field of a traveling wave each time that the segments have a maximum 0- component of velocity, the stream velocity will progressively increase and the electron stream will gain energy from the wave. Such a device is termedan electron accelerator. Thus, by adjusting the axial velocity of the electron stream so that the maximum e-component of velocity of segments of the undulating stream always encounters of decelerating electric field or always encounters an accelerating electric field, a synchronous relationship is established and energy is exchanged between stream and wave.

As has been described previously, an optimum synchronous relationship takes place when the electron stream progresses through a distance equal to one period of the axially periodic radial magnetic field component during a time substantially equal to that required for the electromagnetic wave to progress through a distance equal to such magnetic period plus one wavelength of the electromagnetic wave in waveguide 10, as measured along the length of the waveguide axis. When the tube is opera-ting in this synchronous manner, as the electron stream originally encounters the wave near the entrance of the magnetic unit some stream segments have their H-component of velocity increased and other segments have their O-component of velocity decreased. However, the alternating radial magnetic component periodically converts the entire e-component of stream velocity to a totally z-component as the stream moves down the waveguide. Therefore, the synchronous electron stream contains segments having increased axial velocity and segments having decreased axial velocity. Electron bunches thereby form in the stream in a manner well known in prior art traveling wave tubes. With proper relative wave and stream velocities the undulating electron bunches remain in synchronism with decelerating electric fields of the wave and progressively lose energy to the wave to provide traveling wave amplification.

In the apparatus described, it is required that the electron stream be forced to undulate with a periodic component of velocity parallel to the direction of the electric field component of the electromagnetic wave in order to have an energy-exchanging interaction between stream and wave. Inasmuch as the undulatory motion of the electron stream in FIGS. 3 and 4 occurs in the :9-direction it is desirable that the electromagnetic wave have a predominate portion of the electric field component thereof also parallel-to the Q-direction. The electric field component of such a wave is shown by the dashed circles in FIG. 3, this wave propagating along the waveguide in a mode known as the transverse electric mode. In the transverse electric mode of wave propagation all electric field components of the wave are directed perpendicularly to the direction of wave transmission and therefore may be depicted as lying in transverse planes. Thus, the dashed lines of FIG. 3 illustrate that the electric field components of the wave propagating along axis 11 lie in circular concentric paths about axis 11. The particular circular Waveguide mode shown in FIG. 3 is known as the T13 mode. The TE mode requires the smallest diameter waveguide for propagating a wave of particular frequency wherein the electric field lines lie only in circular paths. Thus, the 0-components of velocity of the undulating electron stream are parallel to the electric field components of the traveling wave employed, so that an energy interchanging action between stream and wave will occur if the optimum conditions for synchronism are satisfied by the relative velocities of stream and wave.

FIG. 3, which is a transverse cross section View of one of the magnetic discs 42, also illustrates the previously mentioned internal longitudinal slots 52 and corresponding lands 53. As previously mentioned, the slots 52 increase the path of the currents in the wall of the interaction waveguide 10 formed by the magnet structure whereby the electric field, represented by the dashed lines of FIG. 3, is increased close to the wall of the waveguide. As shown in FIG. 4, the maximum undulating magnetic field also occurs close to the waveguide wall. Because the maximum electric field and maximum undulating magnetic field now occur in the same region, a substantial increase in interaction efiiciency is provided as compared to the use of a smooth-walled waveguide.

To provide the desired TE mode a first Wave divider 27, shown in FIG. 2, is employed to convert a wave traveling in the conventional dominant mode in a rectangular waveguide to the circularly symmetric 'I'E mode shown in FIG. 3. A similar wave divider 27 converts the amplified circularly symmetric wave received by waveguide sections 19 to a rectangular waveguide mode for transmission to a utilization device. In operation, wave divider 27 receives a wave in the dominant rectangular waveguide mode in an input channel 55. Input channel 55 comprises a conventional rectangular waveguide, the channel dimensions shown in cross-section in FIG. 2 being the narrow dimension of the various rectangular waveguides. The broad dimensions of the waveguides are oriented perpendicularly to the plane of the figure. The energy traveling in channel 55 is divided equally into two equal length waveguide channels 56. The energy in each of channels 56 is again divided equally in two waveguide channels 26. All channels 26 are of equal length. Channels 26 are coupled to respective ones of Waveguide sections 16.

Thus, the energy entering channel 55 is divided into four equal portions, all portions entering the corresponding waveguide sections 16 in like phase and being transmitted through the sections 16 and apertures 24 to waveguide 10. The four separate wave portions entering member 23 from apertures 24 have the electric fields thereof directed in the same circular direction and, therefore, merge in member 23 to provide the required TE wave mode for travel along waveguide 10.

In a manner similar to that described above the amplified wave portions received by apertures 30 and waveguide sections 19 are converted in mode and transferred to an output rectangular waveguide 55.

Thus, the magnetic structure described in connection with FIG. 1 provides a magnetic field component directed perpendicularly to the axis of the waveguide and having extremely short axial periods, wherein the difficulties of flux leakage between adjacent magnets of the structure and the problem of providing individual solenoids for the various magnets is eliminated.

In another embodiment of the instant invention shown in FIG. 5, the magnetic field to be shaped by the various magnetic elements is provided by a plurality of magnetic pole pieces '60 spaced apart along the length of the waveguide, rather than by a solenoid. Pole pieces 60 alternate in magnetic polarity along the length of the waveguide l10. Each pole piece provides a radially directed magnetic field. A north pole piece is disposed between a pair of south pole pieces and a south pole piece is disposed between a pair of north magnetic pole pieces. Pole pieces 60 may be circularly apertured and radially magnetized magnetic discs, and they may be either permanent magnets or the core members of electromagnets.

A plurality of hollow cylindrical magnetic members 61 is disposed along the length of waveguide 10, at least one of such members 61 being disposed between, but spaced from, each pair of adjacent pole pieces 60. Magnetic members 61 are composed preferably of soft magnetic material of the type referred to previously. The soft magnetic members 61 provide effective magnetic short circuits for the magnetic flux provided by the sources of magnetomotive force 60, except in the gaps between each pole piece 60 and the adjacent magnetic members 61. In such gaps a substantial fringing component of magnetic field penetrates into the interior of waveguide 10 and provides the requisite radial component of magnetic field for forcing circular undulatory motion of an electron stream traveling along waveguide 10.

The gaps between the magnetic pole pieces 60 and the soft magnetic members 61 may be filled with a plurality of non-magnetic members 64 to provide the desired vacuum-tight waveguide 10. The structure of this embodiment also may be provided with longitudinal slots, indicated as longitudinal slots 62 and corresponding lands 63 with the advantage of increased interaction efiiciency as discussed hereinbefore in connection with the embodiment of FIG. 1.

The fringing field shown in (FIG. provides an extremely strong third harmonic radial component of magnetic field, the variation of this radial component along the length of the waveguide being shown by the curve designated B in the figure. The length of the period of this magnetic field harmonic is -onethird the period defined by the axial distance between like pole pieces 60. Therefore, an extremely short axial magnetic period for undulating the electrons is provided although employing relatively widely spaced pole pieces, thereby permitting operation with high frequency electromagnetic waves and avoiding the difliculties described heretofore associated with closely spaced pole pieces.

Accordingly, there has been described herein an improved interaction structure for providing an extremely short magnetic period in a device effecting interchange of energy between an undulating electron stream and an electromagnetic wave of very high frequency traveling in synchronism therewith. The short magnetic period is provided not only without reduction in the strength of the magnetic field component required to undulate the electron stream but, in fact, with an increased magnetic field component, thereby retaining a strong amplitude of undulation of the electrons and an effective exchange of energy between waves and electron stream.

The provision of angularly spaced longitudinal slots in the interaction waveguide formed by the magnetic structure results in increased interaction efiiciency by providing a maximum electric field of the electromagnetic wave close to the waveguide wall in the region of maximum undulating magnetic field.

While the principles of the invention have been made clear in the illustrative embodiments, there will be obvious to those skilled in the art, many modifications in structure, arrangement, proportions, the elements, ma terials, and components, used in the practice of the invention, and otherwise, which are adapted for specific environments and operation requirements, without departing from these principles. The appended claims are therefore intended to cover and embraceany such modifications within the limits only of the true spirit and scope of the invention.

What is claimed is:

1. An energy interchange device comprising: means for projecting :a stream of charged particles along an axis; a source of magnetomotive force disposed to provide a magnetic field proximate to said axis and directed substantially parallel to said axis; means forming a waveguide coaxially aligned with said axis, said means providing a spatially alternating component of said magnetic field in a direction perpendicular to said axis for causing said stream to undulate in a direction transverse to said axis, said means forming said waveguide being formed with a plurality of internal slots parallel to said axis; and launching means for launching an electromagnetic wave to travel along said axis wherein an electric field component of said wave is directed perpendicularly to said 2. An energy interchange device comprising: means for projecting a stream of electrons along an axis; a source of magnetomotive force disposed to provide a magnetic field proximate to said axis and directed substantially parallel to said axis; a plurality of alternate magnetic and non-magnetic elements immersed in said magnetic field along said axis, said elements forming a waveguide having a plurality of internal slots parallel to said axis; and launching means for launching an electromagnetic wave in the transverse electric mode to travel along said waveguide.

3. An energy interchange device comprising: means for projecting a stream of electrons along an axis, a source of magnetomotive force for providing a magnetic field proximate to said axis and directed substantially parallel to said axis, a plurality of magnetic elements immersed in said magnetic field and spaced apart along said axis, wherein said elements shape said magnetic field to have components perpendicular to said axis at points along said axis between said magnetic elements: means forming a waveguide coaxially aligned with said axis in the region of said perpendicular components of said magnetic field, said waveguide having a plurality of internal slots parallel to said axis; and launching means for launching an electromagnetic wave in the transverse electric mode to travel along said waveguide wherein the electric field component of said wave is directed perpendicularly to said axis and said magnetic field components.

4. In an energy interaction device, the combination of: a magnetic uni-t provided with an elongated aperture therethrou said unit comprising a plurality of magnetic members having circular apertures therein and means for supporting said magnetic members in spaced apart relationship with the circular apertures thereof coaxially aligned to define said elongated aperture, a source of magnetomotive force for providing a magnetic field, s'aid magnetic unit being immersed in said magnetic field whereby the direction of said magnetic field is substantially parallel to said elongated aperture, an electron gun disposed opposite one end of said magnetic unit for projecting a stream of electrons along the length of said elongated aperture, and launching means for launching an electromagnetic wave in the transverse electric mode to travel along the length of said elongated aperture, said aperture being formed with a plurality of angularly spaced longitudinal grooves.

5. The combination of claim 4 wherein the electric field components of said electromagnetic wave are directed in concentric circular paths, said paths being coaxial with the axis of said elongated aperture.

*6. The combination of claim 4, wherein said magnetic members comprise soft magnetic material.

7. In an energy interaction device, the combination of: a magnetic unit provided with an elongated aperture therethrough, said unit comprising a plurality of axially-aligned, sandwiched, apertured members, alternate ones of said members being magnetic, and the remaining ones of said members being non-magnetic, the apertures of said members defining said elongated aperture, a source of magnetomotive force for providing a magnetic field, said magnetic unit being immersed in said magnetic field whereby the direction of said magnetic field is substantially parallel to the axis of said elongated aperture, an electron gun disposed opposite one end of said magnetic unit for projecting a stream of electrons along the length of said elongated aperture, and launching means disposed near one end of said magnetic unit for launching an electromagnetic wave in the transverse electric mode to travel along the length of said elongated aperture, said aperture being formed with a plurality of longitudinal slots.

8. In anenergy interaction device, the combination of: a magnetic unit provided with an elongated aperture therethrough, said unit comprising a source of magnetomotive force including a plurality of spaced apart apertured elements, wherein [a magnetic field is provided between each pair of adjacent ones of said elements, a plurality of magnetic members having apertures therein, at least one of said magnetic members being disposed between each of said pairs of elements, the apertures of said elements and magnetic members being coaXia-lly aligned to define said elongated aperture, an electron gun disposed opposite one end of said magnetic unit for projecting a stream of electrons along the length of said elongated aperture, and launching means disposed near one end of said magnetic unit for launching an electromagnetic wave in the transverse electric mode to travel along the length of said elongated aperture, said aperture being formed with a plurality of slots along the length of said aperture.

9. The device of claim 8, wherein said magnetic field is oppositely directed between adjacent pairs of said elements.

10. The device of claim 8,-wherein said magnetic elements are uniformly spaced apart, the distance between adjacent ones of said magnetic elements defining a magnetic period, and wherein said stream of electrons is projected with a velocity such that the electrons thereof progress through a distance substantially equal to one magnetic period during a time equal to that required for the electromagnetic wave to progress through a distance equal to one magnetic period plus one wavelength of the electromagnetic wave :as measured along the length of said elongated aperture.

I l. The device of claim 8, wherein higher harmonic magnetic field components are set up in said elongated aperture, and wherein said stream of electrons is projected with a velocity such that the electrons thereof progress through. a distance substantially equal to one period of a predetermined harmonic of said magnetic field components during a time equal to that required for the electromagnetic wave to progress through a distance equal to one such period plus one wavelength of the electromagnetic wave as measured along the length of said elongated aperture.

12. An energy interchange device comprising: means forming an elongated Waveguide, said waveguide being formed with a plurality of internal longitudinal slots; means for launching an electromagnetic wave to travel along said waveguide wherein an electric field component of said Wave is directed transversely to said waveguide; means for projecting a stream of charged particles through said waveguide; and a magnet structure adjacent said waveguide, said magnet structure providing a magnetic field having a first component directed longitudinally along said waveguide and a second component which spatially alternates in a direction transverse to said Waveguide for causing said stream to follow an undulating path through said waveguide.

13. The device defined by claim 112 wherein said magnet structure comprises means providing a longitudinally directed magnetic field and a plurality of spaced maggeiically permeable members immersed in said magnetic 14. The device defined by claim 12 wherein said magnet structure comprises a plurality of spaced magnetic poles of alternating magnetic polarity and a plurality of soft magnetic members each positioned between and spaced from a pair of said poles.

115. The device defined by claim 12 wherein said magnet structure includes a plurality of abutting alternate magnetic and nonmagnet members to form said waveguide, said members being formed with internal angularly spaced notches, said notches being in longitudinal alignment to form said slots of said waveguide.

No references cited.

HERMAN KARL SAALBAOH, Primary Examiner. S. CHATMON, JR., Assistant Examiner. 

1. AN ENERGY INTERCHANGE DEVICE COMPRISING: MEANS FOR PROJECTING A STREAM OF CHARGED PARTICLES ALONG AN AXIS; A SOURCE OF MAGNETOMOTIVE FORCE DISPOSED TO PROVIDE A MAGNETIC FIELD PROXIMATE TO SAID AXIS AND DIRECTED SUBSTANTIALLY PARALLEL TO SAID AXIS; MEANS FORMING A WAVEGUIDE COAXIALLY ALIGNED WITH SAID AXIS, SAID MEANS PROVIDING A SPATIALLY ALTERNATING COMPONENT OF SAID MAGNETIC FIELD IN A DIRECTION PERPENDICULAR TO SAID AXIS FOR CAUSING SAID STREAM TO UNDULATE IN A DIRECTION TRANSVERSE TO SAID AXIS, SAID MEANS FORMING SAID WAGEGUIDE BEING FORMED WITH A PLURALITY OF INTERNAL SLOTS PARALLEL TO SAID AXIS; AND LAUNCHING MEANS FOR LAUNCHING AN ELECTROMAGNETIC WAVE TO TRAVEL ALONG SAID AXIS WHEREIN AN ELECTRIC FIELD COMPONENT OF SAID WAVE IS DIRECTED PERPENDICULARLY TO SAID AXIS. 